1. Field of the Invention
The present invention relates to transistor amplifier circuits and, more particularly, to class-A differential amplifier circuits.
2. Description of the Related Art
One of the most common transistor amplifier configurations used in amplifier circuit design is the differential amplifier, an example of which is shown in FIG. 1. Two input signals are applied to terminals 1 and 2, respectively, and are amplified by transistors T1 and T2, respectively. Both transistors are driven by current source 20. The output signal presented across terminals 4 and 5 is an amplification of the differential signal applied across input terminals 1 and 2. In this design, both transconductance (g.sub.m) and maximum output current are directly set by the tail current source 20 (maximum output current is defined as the maximum differential current available from the output of a differential amplifer when the inputs are unbalanced enough to cause all of the tail current to flow out of one side of the circuit). In order to achieve flexibility in design (i.e. to obtain independence of these two design parameters), emitter degeneration is commonly used as shown in FIG. 2. Emitter resistors 30 and 32 set an upper bound on transconductance, independent of the emitter current (i.e. the maximum output current).
It should be noted that although all of the figures illustrated herein show amplifiers constructed with bipolar transistors, the concepts can be easily extended to allow the use of any controllable amplification device. For example, the use of JFETs or MOSFETs in the differential amplifier gives performance similar to degenerated bipolar devices due to their inherently lower transconductance.
When an active load is connected to a differential amplifier, the input-referred voltage noise of the active load is defined as the noise current of the load divided by the input stage transconductance. Input-referred voltage noise of low transconductance input stages is usually dominated by the load, since its noise currents increase proportional to the square root of its collector current. Input-referred voltage noise can become quite large when the input stage transconductance is lowered by degeneration, since there is no tracking of this transconductance and the noise current of the load.
Another prior art topology, shown in FIG. 3, uses complementary transistors and resistors to make the maximum output current limited only by the differential input voltage, allowing some independence of maximum output current and transconductance. Unfortunately, base-emitter characteristics of complementary transistors do not track in practice and therefore T5 and T6 collector currents are unpredictable. Also, it may be undesirable not to set a maximum output current independent of the differential input voltage levels.
Operational Amplifiers (Op Amps), for instance, are designed to be slew rate limited by a capacitance charged by the maximum output current of the input stage differential amplifier, and this limits the maximum rate of change of voltage on each node in the circuit. Since Op Amps are biased by current sources rather than by voltage, the problems of bias circuit recovery, as discussed in U.S. Pat. No. 4,879,524, become prevalent and may even lead to complete shutdown of the bias circuit. The input-referred voltage noise is roughly the same as that in the circuit of FIG. 2.
Class-B differential amplifiers, such as those shown in FIGS. 4 and 5, switch a low quiescent current into a large maximum output current. The main advantage of this design is that a higher input stage transconductance can be used to reduce the input-referred voltage noise of the load. There are also, however, several disadvantages such as unpredictable switching thresholds and a maximum output current that is difficult to control. Since the maximum output current can be very large, extra capacitance must be added to limit the maximum rate of change of critical node voltages, as previously discussed with reference to FIG. 3. A particular disadvantage with the circuit of FIG. 5 is that the output is not double-ended, thus restricting general use.
Therefore, it can be seen that the need exists for a differential amplifier design that allows for independent control of input stage transconductance and maximum output current without any of the limitations of the previously discussed prior art circuits.